Privacy signaling system



l l I l l I lull.

2 Sheets-Sheet 1 E. s. PURINGTON Filed Noy. 17, 1938 INVENTOR ELLISON S.PU GTON ATTORNEY PRIVACY S IGNALING SYSTEM June 11, 1940,

7 u E. s. PURINGTON PRIVACY SIGNALING SYSTEM Filed Nov. 17, 1938 2Sheets-Sheet 2 INVENTOR ELL/SON S. FUR/N TON AT I'ORNEY Patented June11, 1940 UNITED STATES PATENT FFICE to John Hays Hammond, Jr.

Application November 17, 1938, Serial No. 240,898

13 Claims.

The present invention relates broadly to privacy signaling" systems andmore particularly to a privacy system of radio telephony using a maskingsignal.

It is an object of the present invention to devise a privacy system oftelephony which has special and marked advantages over the usual typesof privacy systems.

Generally speaking, heretofore there didnot exist a privacy system oftelephony which did not give some indication of the fact that telephonywas in progress. This disadvantage has been overcome with the presentinvention by concealing the true message with a false telephonic signalor message which is on the same carrier wave as the privacy message. Inthis way the false message, so to speak, masks the presence of the truetelephonic signal. In accordance with the present invention, use is madeof duplex transmission, with the true signal differing in spectralcharacteristics from the normal type of telephonic signal by which themasking signal is conveyed.

V Briefly outlined, the present invention involves the following steps:

' (l) Modulating a carrier wave by a masking signal in the normal mannerof usual radio telephony. H (2 Converting the speech frequencies of thetrue signal to a band of frequencies outside the band used by themasking signal, and modulating the radio carrier also by the modifiedfrequencies. (3) Varying the carrier, if necessary, at a low rate ofspeed so that side bands of the carrier wave change from one position toanother, whereby any normal type receiver adjustment which might revealthe true signal cannot be maintained.

' (4) Reception to produce currents corresponding to the modulation ofthe carrier, selection of those currents representingthe true message,and reconversion of these currents to the initial ar rangcment to giveintelligible indication.

In an actual circuit set up which produced good results, the true signalwas sent by converting from the speech range 200-2500 to the modulatingrange 6200 to 3900 approximately, and the mask signal remained in theband 200 to 2500. so This was a conservative choice to give a convertedsignal completely outside the original speech band, fairly completelyfree from harmonies of the original speech band, and also to give a goodquality masking signal. .It is be- .15 licved the total audio band widthof 6200 cycles which was used in the circuit is greater than isnecessary or desirable, and that the system can be well operated usingmodulating frequencies not in excess of 4500 cycles. This would some owhat increase the audibility of the sound produced by beating of thecarrier with the side bands, but would also increase themasking effectdue to the masking signal.

The masking signal need not be of good quality, and in fact it isprobably better that the masking signal be of a garbled nature withmeaningless sounds, such as produced by a phonograph disc operated inthe reverse direction. In this way a saving in band width can beefiected chiefly by cutting down the masking signal to a range of say50-1000 cycles, permitting a true message using modulation frequenciesin the range 1500- 4400 cycles, converted from the range 100-3000cycles. With this arrangement there is an overlap of the originalmessage band, and the band "to which it is converted, with some chanceof loss of quality due to imperfections in conversion. However, thisshould not be a disadvantage since one should not expect as high qualityin privacy systems as is possible in normal telephony, due to thedifliculty of providing perfectly sharp cutoff in converter filterswhich usually results in a deficiency of low notes. Moreover, a lowquality signal is more easily concealed than a high quality signal,therefore, the user who finds it convenient 25 to use a privacy systemmust expect asomewhat lower grade of communication than if normaltelephony were used.

While the inventionhas been outlined above in a general manner, it isthought that the nature of the invention will be more readily understoodby a more detailed discussion of a particular one of many possibleembodiments thereof such as that shown in the accompanying drawings,wherein,

Fig. 1 is a simplified circuit diagram of a transmitter system which maybe used for transmit ting signals in accordance with the present in- Vvention;

Fig. 2 illustrates a receiver which may be used for receiving signalstransmitted from an arrangement like that shown in Fig. 1; and, I

Figs. 3 and 4 are diagrams used to explain the character of thetransmitted signals.

Referring particularly to Fig. 1, the terminals l and 2 are arranged tobe connected to a suitable source of speech currents such, for instance,as a microphone or pre-amplifier (not shown). The signal energyimpressedacross terminals l and 2 may be transmitted to the transmittingantenna and ground arrangement shown at l05--l06 through two pathsdepending upon whether or not privacy transmission is desired. For. thispurpose two switches 6, I, 8 and 9, In, H are provided and arranged sothat when switch blade 6 is in contact with terminal I and switch blade9 is in contact with terminal l0, terminals I and 2 are directlyconnected to terminals 3 and 4 respectively, whereas, when switch blade6 is in contact with contact point 8 and switch blade 9 is in contactwith contact point i I, the terminals 0 quency multiplier circuit 3M.

i and 2 are connected to terminals 3 and t through the privacy circuits,shown at it, it and I3 and comprise a converter circuit, a signal filter(band pass) and a mask circuit respectively. It is seen, therefore, thatthe arrangement shown in Fig. 1 is constructed so that the privacycircuits may be cut in or out by means of the pair of switches 6, 1, 8and 0, 50, ii. The terminals 3, i are connected to the input terminalsHM, I02 of a radio telephone transmitter which may comprise an audioamplifier 302 and a modulator and radio power amplifier 303, which feedsinto the antenna ground system i05, I06.

The carrier frequency is generated by means of an oscillation generatorcircuit including an oscillator tube I09. The output of the-tube may befed directly into the modulator amplifier 303 or to the modulatoramplifier 303 through a fre- Switch 300 is for the purpose ofdetermining the path 'chosen.'

The oscillation generator may be operated either as a crystal controlledoscillator in which case switch blade i it is placed in contact withpoint H0 thus connecting crystali 2i in the circuit, or as a variablefrequency oscillator in which event switch blade I i8 is placed incontact with switch point I 20 thereby removing the crystal I2! from thecontrol circuit and connecting in its place frequency varying means tobe described hereinafter.

With the switches arranged so that arm 6 contacts pole 8 and arm 9contacts pole ii, the signal currents impressed across the terminals iand 2 are fed to the converter circuit 52 through an input attenuator 22-2 3 and an input" filter 25-32, and altered by the converter, purifiedby the signal filter id, and combined with masking signals from themasking circuit 83, from which the higher frequencies are removed by themasking filter it. After combining, the converted signals and themasking signals pass through terminals Sand d and are used to modulatethe transmitter arrangement 302, .803 and transmitted by the antenna andground system M5, 06. The convertercircuit i2 incorporates a pair of lowpowered pentodes 36 which may be of any well known type such ascommercial type 57 or 6C6 and one oscillator tube 3?, which may be acommercial type 56 or "76. The signal frequencies to be converted areimpressed upon the first or control grids of tubes 36, 38 from thesecondary of a transformer 33 while voltage from the oscillator circuitis'impressed upon the third or suppressor grids of the tubes 35. Theanodes. screen grids and cathodes are connected in parallel and theoutput is derived from between anodes and anode battery supply.

Recommended values of currents and voltages for a circuit using thetubes mentioned above are approximately:

S u ppr e ssors ground or zero potential, no suppressor current ControlMax. volts between control grids 3.5 volts A. C.

Max. volts between suppressorgrids 17.0 volts A. C.

These should give an output voltage of about does not pass into theoutput. lower side bands are thus produced without the The idealrelation between plate current and electrode voltage for a tube for useas a converter in this type of circuit is:

tioned above, although there are higher order terms especially for thesuppressor voltage. Al-

though the characteristics for the grid control voltage could beimproved artificially, the output is fairly free from harmonics of theoriginal frequencies, and those that pass through the modulating systemwill not seriously disturb the speech quality unless the circuitis-excessively forced. There are other more. complex methods of avoidingharmonics of the original frequencies passing unconverted through theoutput system, but it is not believednecessary to use them.

The oscillator circuit shown is of the simplest type suitable for thepurpose. For best results it should be constructed in such' a manner asto avoid capacity effects through the coupling. In the arrangement shownthe anode or plate of oscillator tube 31 is connected to the terminal ofa suitable power supply system, shown as a resistor assembly I6, througha plate coil 391 which is coupled to grid coil it! connected be tweenthe grid of tube 31 and ground. An oscillator or tuning condenser 52 isconnected between the plate and grid of tube 37 and acts to determinethe frequency of the generated oscillations. A by-pass condenser M isconnected between the terminal I and ground. The cathode of theoscillator tube is grounded through a condenser and bias voltage issupplied to the tube by connection of this last named condenser acrossterminals h and g of the resistor assembly it.

The circuits of tubes 38 are provided with balancing potentiometers 35and M in order that corrections may be made for slight inequalities ofthe tubes. A phone jack 45 provides for checking the balance, byobservation of the currents flowing to the plates from the plate supplysource. By adjustment of the potentiometer 35 the original speechfrequencies are excluded from the output of the converter i2 and byadjustment ofthe potentiometer M the oscillator frequency The upper andpresence of the original currents, although second harmonics produced bythe tubes also pass into the output. In the event that the oscillatorfrequency is very high or the transformers 33 and 88 are notsufiiciently free from capacity effects, it may be desirable to usetrimmer condensers such as 90, 97, 98 and 09 for improving the balance.

The input filter system is made up of reactors 25-32 which serve tochoose the band of frequencies which are to be converted, and in thisway free the tubes 30 from having frequencies impressed thereon whichcannot be utilized.

The converter output system as which is tuned broadly to the band offrequencies to be used,

comprises a ballast resistor 52 shunted by a primary condenser 50 acrosswhich is connected a primary choke t8 and the primary winding of atransformer 41, the secondary of the transformer 4'! is connected inseries with a secondary choke 48, across the secondary condenser Theoutput system 46 is arranged so that the voltage is stepped down bymeans of a trans-- former 41 to permit most of the filtering to be doneat a lower impedance level.

Before the converter input filter -32 and I after the converter outputsystem 46 there are connected attenuators for providing the bestadjustment of this circuit for voltage input and output. Theseattenuators comprise the impedances 22, 23 and 24 connected before theinput filter and impedances 53, 54 and 55 connected after the outputsystem 45. These imp'edances may be fixed after experiment. The powerlevel at the input terminals I, 2 should preferably be such that theinput to the tubes is optimum without using any artificial attenuator.If this is done no amplifiers preceding the converter or subsequent tothe converter will be necessary.

Thesignal filter into which the output from the converter output system46 is fed in its entirety a band pass filter, made up of a low passportion 5B-66 in series with a high pass portion 61-18. Whilea systemmade up of band pass sections may be used, it is believed that thearrangement disclosed is preferable to a system made up of band passsections because of the high ratio of maximum to minimum frequencies,and because of the great ease of adjusting the upper and lower cut-oilcharacteristics. For systerns using speech inversion the high passcut-oh? characteristics are not so important at the transmitter as arethe lower pass cut-off. The low pass cut-off slightly below thefrequency of the oscillator should bewquite sharp. If too dull, thequality of the transmission will suffer due to the fact that some of thefrequencies of the undesired sideband get through the system, and

due to poor transmission of the low speech frequencies.

In some applications, the oscillator 31 may be given a choice of twofrequencies, either slightly below or slightly above the band to whichthe converter output system 46 and signal filter I4 are adjusted. Inthis case, both portions of the filter must provide sharp cut-off.

The mask circuit I3 is indicated as a phonograph turntable 80 driven bya motor 19 through connection. thereof to a suitable power supplysystem. It is to be understood that any other source of maskingfrequencies could be used and that the invention is not to be limited tothe use of a phonographic device. The phonograph pickup device Bi feedsinto a mask filter I5 through an attenuating circuit made up ofimpedances 82-44, and then through the filter circuit I5 which is madeup of the reactors 85 through 95. The filter circuit I5 is arranged forpassing frequencies lower than those transmitted by the'signal filter.It may be desirable for some purposes to include-masking frequenciesalsohigher than those of the signal filter in which case the mask will be onboth sides of the signal giving more effective concealment.

The outputs of the signal filter I4 and the masking filter I5 are joinedtogether and fed to the output switch pole II, preferably throughreactors 16, I1, 18 and 98, 94, 55 respectively, suitably designed toavoid reaction effects of one filter upon the other. I

A meter 5 is provided for checking the. signal strengths of thetransmitter input to obtain best transmission coiitlition. While thesignal and mask maybe made of approximate equal intensity, if theequipment is of sufilcient quality and of suflicient power there will beincreased advantage as to privacy if the mask is made of much greaterintensity than the signal. The

limit is reached when the transmission of the 5 privacy is too weak tobe intelligible due to various reasons, as, for instance, cross talkfrom the masking circuit. It has been found that cross talk can resultfrom many causes amongst which are incomplete filtering of the maskedfilter, production of harmonics of the masking frequencies by the radiotransmitter subsequent to the masked filter, non-linearity of detectionat the receiver, and incomplete filtering at the receiver.

The improvements in radio transmitters and 15 receivers with respect tolinearity of modulation and demodulation'is a contributing factor tomaking the masking circuit practicable under modern conditions, whereas,it might not have been practicable during the earlier developments ofradio telephony,

In some applications, it may be desirable to create serious crossinterference upon highly selective radio receivers and thereby furtherconceal the presence of the privacy signal. This may be accomplished byan artificial frequency modulationof the radio transmitter.

Distortions in the audio amplification would result in harmonics-of themask frequencies entering into the signal band. Distortion of modulationwould result in second and higher order side bands of the maskingcircuit being coincident with portions of the signal side bands. Crosstalks from these sources with modern designs will be sufiiciently smallunless the signal 35 side bands are very weak in comparison with themask side bands. However, if the signal sidebands are much greater thanthe mask side bands, the presence of the privacy signal may be revealed,due to insuificient masking, although not necessarily resulting inintelligibility. The use of artificial frequency modulation is toprevent loss of privacy by continuous wave reception with the localheterodyne frequency set properly at zero beat. If this artificialfrequency modulation is required, it may be desirable to simulateprivacy transmission on the mask circuit, as, for example, by use ofcode words, or by simple speech inversion, or by running a phonographrecord backwards, so that the real privacy signal may be overlooked.

The artificial frequency modulation may be accomplished most easily bysubstituting a tank circuit for' the crystal of a crystal controlledtransmitter. In the drawings, Fig. l, Ili9 represents the masteroscillator tube, with output tank circuit H0, III delivering energydirectly or through a frequency multiplier circuit 3M to the higherpower radio frequency circuits 303 of the radio transmitter. Thefrequency generated by the oscillator I09 is controlled mainly bycircuits in a constant temperature oven, normally a crystal device I 2|which when point I I8 is connected to point II9 oscillates by energy fedback from the plate tank circuit III), III through the plate-gridcapacity I". With the points H8 and I20 connected together, however, thefrequency'is controlled by inductors and condensers- I23--I I8 and lowcapacity variable condensers beyond point I21 which may be external tothe constant temperature chamber or oven. with the points III-I28connected together, the frequency may be shifted by operating the keyI34 which changes the capacity by a slight amount. It will be noted thatthe key I34 when depressed 15 energizes the electromagnetic coil E83 byconnection thereof across the battery 35. This in turn closes the relayswitch I32 which in effect connects the'condenser I30 across condenserl3l.

and driving the wobbler rotary disc l3! and' cams Hi2 and M3 by geardevices not shown specifically. The Wobbler rotary disc l3? shouldrotate preferably at a slow speed in order to give a fundamentalfrequencyof wobble of between 5 and 50 cycles, and the cams H32 and M3which cut the fixed condensers I38 and I39 respectively in and outshould also be operated at slow speeds. The mean frequency with theWobbler operating when the switch arm 52? is connected to point H29should be the'same as the mean frequency with key relay 932 operatingand arm i2! connected to the contact I28. To make this adjustment therehas been provided a variable condenser l3i. Also the mean frequencyshould be equal to thecrystalfrequency, and this is easily achievedbysetting a monitoring receiver for zero beat with the crystal operating,and then with the switch thrown to connect switch arm M8 to the contactE20. by adjusting the Vernier condenser i26 so that the same toneresults in the monitoring receiver with key I34 either up or down. It isbelieved that a wobbler system such as the one described above should beprovided in any high grade crystal control transmitter intended forprivacy, to allow for adaptation to service with a wobbler and easychecking of performance.

For convenience the power supply for the arrangement shown in Fig. 1 hasbeen shown generally as a resistor assembly iii. The points a through iof the resistor assembly are connected to the various points indicatedin the remaining portion of the diagram by the same letters.

Broadly speaking, the receiver for the system may be of any generaltype, however, it is essential for minimum of cross talk that thecircuits be linear up to the detector, and that the detections belinear. The selectivity should not be less than that found in a goodhigh fidelity receiver. In case a Wobbler is used at the transmitter,excessive selectivity at the receiver would cause variations of thereceived signal due to detuning, but'because of the low frequency of thewobbler devices suggested above, this will produce no serious audibleeffects.

Referring now more particularly to the receiver arrangement shown inFig. 2, it will be seen that there is incorporated therein the usualantenna 2! which feeds into the preliminary circuits 204 involvingstandard practice of a. radio receiver. The output of 205 appears acrossthe winding 205 to which is coupled a winding 205. The windings 205 and205 represent the last radio frequency or intermediate frequencytransformer, by which signals representing the transmitter radiationsare impressed upon the diodes of a duplex-diode triode tube 208. Tube208 and its related circuits represents a standard and preferable formof detector device, and may include circuits also for automatic volumecontrol to establish best signal strength conditions for the operationof the detector and other circuits. ,Such automatic volume controldevices have not beendisclosed in the diagram nor described in detailsince they may be as a matter or fact incorporated in other circuits ofthe standard part 204. Furthermore such devices are well known to thoseskilled-in the able by the use of resistor H5 and stoppage condenser 2l8whereby the output of the rectifier amplifier device 208 is delivered tothe low pass filter composed of reactors 2l9-223. This filter should bedesigned so as to pass currents below the highest frequencies used tomodulate the transmitter, and may be used also in the reception ofnormal telephonic signals.

For normal reception, switch points 222 and 225 should be connectedtogether and switch 223 and 22! should be connected together whichpermits the signals to pass directly from the filter 2l9-223to the inputof the power amplifier tube 233 in the output of which there isconnected a loudspeaker arrangement 238-2 50 or any other desiredindicator. The connection between the output of the power amplifier tube233 to the osidspeaker is through the output transformer For receptionof the privacy signal, the switch points 224 and 226 are connectedtogether and switch points 221 and 229 are connected together therebycutting in a reconverter circuit and filters. The input attenuators241-243 and the output attenuators 2l9-28I may be fixed or with properdesign omitted entirely from the circuit. The, filter devices, 244-253in connection with the filter 2l9-223 constitutes a band pass system forselection of the signal frequencies and rejection of the maskingfrequency. The transformer 254 may be designed to assist further in thisseparation. I

The output of the filter 244-253 feeds into the input of a pair ofreconverter tubes 258 through the transformer 254. .An oscillationgenerating circuit including oscillator with the reconverter circuit.The reconverter tubes 258, oscillator 259 with all associated circuitsmay be identical with those used at the transmitter and incorporatingthe tubes 35 and tube 31 (see Fig. 1) except that the input balancer maybe omitted if desired, as shown. A balanced detector of this type ispreferable in order to insure complete exclusion of the heterodyne tonesfrom the subsequent circuits. The detector output stepped down by theauto-transformer 268 operates through the output filter 2H-2l8 to purifythe signals, and further aids in eliminating themasking tones. Thesignals now reconverted to the same signals as those operating theconverter at the receiver, except for cutting by the is now impressedupon the power tube 233 and further amplified for indication by asuitable indicator device represented generally in Fig. 2 by theloudspeaker combination 238-240.

A clearer idea of the invention will be had from a. study ofthe diagramsshown in Figs. 3 and 4 wherein Fig. of the mask and signal audio energyin the signal amplifier and modulator before itis used to modulate thecarrier energy. and Fig. 4 illustrates the relative arrangement of themask and signal energy with respect to the carrier, that is, aftermodulation of the carrier in the power amplifier output.

tube 259 is associated 3 showsv the relative arrangementItwiilbeseeniromFlgAtbatthemask 7o detecting the transmitted energy andderiving energy is used to modulate the carrier wave in ordinary fashionand that, therefore, an ordinary receiver tuned to the carrierv wouldreceive the mask sidebands. The privacy signal on the other hand hasbeen displaced in frequency as indicated by the displaced signalsidebands in Fig. 4 so that the ordinary receiver would, of course, notreceive the privacy signals.

. Although a specific embodiment of the invention has been shown forpurposes of illustration, it is to be understood that the invention iscapable of various uses and that various changes and modifications maybe made therein as will be readily apparent to a person skilled in theart. It is intended that the invention be limited only in accordancewith the following claims when interpreted in view of the prior art.

I claim:

1. A method of signaling which consists in generating a carrierwave,.producing a first band of audio frequencies representative of aset of signals, producing asecond band of audio frequenciesrepresentative of another set of signals, converting the last named bandof audio frequencies to another band of related frequencies,simultaneously amplitude modulating the carrier wave in accordance withboth said first band of audio frequencies and the converted band offrequencies, transmitting the resultant carrier and upper and lower setsof side bands, inter.- cepting the transmitted energy at a receivingpoint, detecting the intercepted energy to derive therefrom the signalcomponents, separating out from the resultant energy the signalcomponents due to modulation ofthe carrier by the first band of audiofrequencies and the signal components due to modulation of the carrierby the converted band of frequencies, reconverting the last named signalcomponents to a band of related audio frequencies similar to the secondband of audio frequencies and deriving the seccond set of signals fromthe reconverted energy.

2. The method described in the next preceding claim characterized bythat the second bandof audio frequencies is also frequency inverteddur-' ing the conversion process at the transmitter and re-invertedduring the reconversion process at the receiving point.

3. A method of privacy telephony utilizing multiplex telephonictransmissions characterized by that one of the multiplex transmissionsproduces intelligible signals in the normal type of telephonic receiverand another produces unintelligible signals in the normal type ofreceiver but produces intelligible signals in a specially designedreceiver, said second-named transmission constituting the messagedesired to be conveyed and said first-named transmission constituting afalse or masking message to discourage search for the true or hiddenmessage.

4. In a privacy telephonic system, means'for generating carrier waveoscillations, means for producing masking signal energy, means forproducing the signal energy constituting the message desired to beconveyed, means for altering the characteristics of the message signalenergy in a predetermined reversible manner, to thereby renderintelligible reception of the message signals possible only in apre-arranged manner, means for modulating the carrier oscillations byboth the masking signal energy and the altered messagesignal energy,means for transmitting the modulated carrier oscillations, a receivingsystem including means for intercepting and therefrom the masking signalenergy and the altered message signal energy, means for separating outthe altered message signal energy and altering the characteristicsthereof so as to bring the energy back to its original form and meansfor utilizing the resultant energy.

5. In a radio telephonic system, the method of imparting a high degreeof secrecy to signals desired to be conveyed from one point to another,which method consists inproducing a single carrier wave, simultaneouslymodulating the carrier wave by both the desired signals and byintelligible masking signals, the masking signals being characterized bythat the interception and detection of the modulated carrier by a normaltype of radio telephonic receiver produces only the masking signals, thecharacteristics of the desired signals being altered so that translationthereof is possible only in a specially designed radio telephonicreceiver, the masking signals constituting a false message calculated todiscourage search for the true or hidden message.

6. In a privacy telephony system, means for producing a band of audiofrequencies for masking purposes, means for producing a band ofaudiofrequencies for signaling purposes, means for converting the lastnamed band of frequencies to a band of frequencies which is differentthan the first named band of audio frequencies, means for producing acarrier wave, means for modulating the carrier wave by both the maskingenergy and the converted signaling energy, means for transmitting thecarrier wave thus modulated, a receiving system including means forintercepting the transmitted energy, a first detector circuit coupled tothe last named means, for deriving from the intercepted energy themodulation components thereof, means for'separating out the maskingenergy component and the converted signaling energy, means forrecoverting the last named energy to produce therefrom the signalingenergy and means for impressing the resultant signalingenergy upon a'utilizing circuit.

7. The steps in a method of secret signaling which comprise generating acarrier wave, producing two audio frequency signals one of which is thetrue signal and the other a masking signal, both of said signalsoccupying the same audio frequency band, generating oscillations offixed frequency, combining the generated oscillations with the'trueaudio frequency signals to produce from said combination a band offrequencies representative of the true signals but outside the firstnamed audio frequency band, generating carrier wave oscillations,modulating the last named carrier wave oscillations simultaneously byboth the masking signals and the last named band of frequencies, andtransmitting the modulated carrier, wave oscillations, receiving thet'ransmittedenergy, detecting the same to derive therefrom themodulation components thereof,

selecting therefrom the modulation componentsdue to the last named bandof frequencies, gencrating oscillations of a frequency of such a valuethat whencombined with the selected band of frequencies the originaltrue signal audio frequencies are produced and combining the generatedoscillations with said selected band of 40 modulator for combining thetrue signal energy 55 respect to the cathodes thereof, a. source ofsigdesired to be transmitted, means for separately 7 producing a band ofaudio frequencies representative of'a second set of signals desired tobe transmitted, means for converting said first band of audiofrequencies to a corresponding but differant and inverted band offrequencies, means for producing carrier wave oscillations, means formodulating the carrier wave oscillations simultaneously by both theconverted band of freprising the band of audio frequenciesrepresentative of the second set of signals, said other trans= missionnetwork including means for reconverting the other modulation componentto produce therefrom a band of audio frequencies having substantiallythe identical characteristics as the first named band of audiofrequencies and transmitting the reconverted band of frequencies to theutilizing circuit.

9. A communication system as described in the next preceding claimcharacterized by that means are provided atthe transmission end for.wobbling the produced carrier wave oscillations in a predeterminedmanner.

10. In a privacy telephony system, means for producing two audiofrequency signals simultaneously, one of said signals being the truesignal and the other thereof a, masking signal, a source of localoscillations, means comprising a balanced and the oscillations from saidsource, said balanced modulator being arranged to eliminate from theproducts of said combination both the true signal energy and the sourceoscillations, means for generating a. carrier wave and means forsimultaneously modulating the carrier wave by the products of said lastnamed combination and the masking signals.

11. A balanced push-pull modulator circuit comprising a pair ofelectronic tubes each thereof being provided with an anode, a cathodeand at f least two grid'electrodes, a connection between the cathodesthereof, means for negatively bias ing both grid electrodes of each ofsaid tubes with naling energy, a source ofoscillations, an input circuitincluding a transformer provided with primary and secondary windings,means for connecting the source of signaling energy across the primarywinding, means connecting the secondary winding between a grid of one ofsaid tubes and the corresponding grid of the other of said tubes. aresistance circuit connected between said twoaaoaoso grids, a variabletap on said resistance circuit, means for connecting said tap to saidfirst named connection, a second input circuit including a coupling coilconnected between the other corresponding grid electrodes of said'tubesa balancing circuit connected across' said coupling coil, means forelectrically connecting said source of oscillations across said inputcoil, an output circuit for said modulator comprising a commonconnection between each of said anodes and said cathodes and includingan output impedance device, a utilizing circuit and means for couplingthe utilizing circuit to said output circuit.

12., The steps in a method of telephony utilizing multiplex telephonictransmissions which comprise'generating a carrier wave, producing afirst band of frequencies representative of a set of signals, producinga second band of frequencies representative of another set of signals,converting the second named band of frequencies into a third band ofrelated frequencies which is displaced from said second band by apredetermined amount, simultaneously modulating the carrier wave by boththe first band of frequencies and the third band of frequencies,transmitting the resultant energy, intercepting the transmitted energyat a receiving point, detecting the intercepted energyto derivetherefrom the signal components, separating out from the detected energythe signal components due to modulation of the carrier by said firstband of frequencies and the signal components due to modulation of thecarrier by the third band of frequencies, reconverting the last namedsignal components to a band of related frequencies similar to the secondband of frequencies and deriving from said reconverted energy the secondset of signals.

13. The steps in a methodof multiplex telephony which comprisegenerating a carrier, producing a band of audio frequency signals repf,resentative of a message to be transmitted, producing a second band ofaudio frequency signals representative of another message to betransmitted, producing from the second band of audio frequency signals aband of frequency inverted signals which are displaced in the frequencyspectrum from the first band of audio frequencies by a predeterminedamount, simul-. taneously modulating the carrier by both the first bandof frequencies and the bandof frequency inverted signals, transmittingthe resultant energy, intercepting the transmitted energy at a receivingpoint, detecting the intercepted energy and deriving therefrom thesignal components, separating out from the detected energy the signalcomponent due to modulation of the carrierby the first band offrequencies and the signal components due to modulation of the carrierby the band of frequency inverted signals, and producing from the lastnamed signal-components a. band of inverted frequencies which isdisplaced in the frequency spectrum backto the position of the secondband of audio frequencies.

ELLISON S. PURINGTON.

